ABSTRACT Pinnacle reef tracts are geomorphic features of carbonate systems that originated in the early Silurian and display an episodic distribution into the Cenozoic. Detailed study of Silurian pinnacle reefs of the United States midcontinent demonstrates repeated motifs, but most enigmatic is the coincidence of carbonate carbon isotope (δ 13 C carb ) excursions and reef pulses. Silurian δ 13 C carb excursions were associated with environmental changes and extinctions, and reefs appear to mark a resurgence of conditions favorable to biomineralizers following those extinction events. Previous workers in the region identified six discrete reef origination events in the United States midcontinent during the Silurian. Our reevaluation of outcrops and cores, conodont collections, and the generation of considerable new chemostratigraphic data across the region are clarifying the age relations of these events and their relationships to perturbations of the global carbon cycle.

Abstract The Niagara-Lower Salina reef complex reservoirs of the Michigan Basin host significant hydrocarbon volumes and have recently been identified as promising targets for enhanced oil recovery and carbon sequestration. Although these carbonate buildups have been studied extensively since the late 1960s, there is still wide uncertainty and disagreement concerning their morphology and internal stratigraphic and facies architecture. The prevailing paradigm depicts the reef complexes as tall, symmetric “pinnacles” with heterogeneous internal facies distributions that are patchy and unpredictable. The current study challenges this model of the reefs by examining four Silurian reef reservoirs with abundant core and petrophysical wire-line logs. New and existing subsurface data show that Silurian reefs in the Michigan Basin are highly asymmetric with internal facies distribution patterns that are strongly influenced by east-northeast paleowind direction. Six major depositional environments are identified during the main stage of reef complex growth based on sedimentological characteristics observed in core, as well as the vertical progression (stacking) of facies observed both in core and wire-line log signatures. A central reef core environment is identified based on interspersed coral-stromatoporoid boundstone and skeletal wackestone facies consisting of frame-building organisms such as tabulate corals and stromatoporoids, as well as intrareef faunal assemblages of bryozoans, brachiopods, crinoids, and rugose corals. Environments to the east (windward) of the central reef core are steeply inclined to the east (~40°) with narrow facies belts characterized by coarse reef talus. In contrast, environments to the west (leeward) of the central reef core have shallower slopes that dip to the west (< 15°) and are characterized by wide facies belts composed of carbonate mud and skeletal debris that become finer and thinner in the leeward direction. Application of this new Silurian reef model to reef complexes throughout the basin demonstrates remarkable consistency with respect to the overall asymmetric shape of the reef complexes, as well as the windward-leeward internal facies architecture. The asymmetric architecture and windward-leeward facies distribution patterns described in the new model offer a significant improvement upon preexisting models for Silurian reefs in the Michigan Basin and more accurately reflect our modern understanding of how environmental controls affect reef development and architecture. Furthermore, this new reef model can be used to more accurately predict the shape and internal facies distributions for other Silurian reef complex reservoirs within the Michigan Basin, particularly those that lack abundant well control.

Abstract As oil imports in the United States approach 60% of total daily consumption, more efforts are being expended to maximize recovery from known domestic oil fields. As part of this effort, CO 2 flooding of reservoirs has been proven to be an effective means to increase the recovery of oil bypassed during primary production, albeit commonly at significant cost because of capture, compression, and transportation of adequate CO 2 . At the same time, global and national interest in the viable geological sequestration of anthropogenic CO 2 , a major greenhouse gas when emitted into the atmosphere, is also becoming more significant. In the Michigan Basin, the juxtaposition of the Devonian Antrim Shale natural gas trend, one that contains high levels of associated CO 2 , with the mature Niagaran (Silurian) reef oil play, characterized by reservoirs with high percentages of stranded oil, may provide an economically viable model to combine enhanced oil recovery (EOR) efforts with the geological sequestration of CO 2 . Niagaran pinnacle reefs in the Michigan Basin have produced more than 450 MMBO since the late 1960s. Because of the complex heterogeneity of the reef reservoirs, however, primary production averages only around 30% with secondary waterflood programs typically capturing an additional 12%. The northern reef trend in the Michigan Basin comprises an immense hydrocarbon resource, located in hundreds of closely spaced but highly compartmentalized reef fields in northern lower Michigan. These geologically complex carbonate reef reservoirs present not only significant opportunity for EOR operations because of known traps, quantifiable remaining oil, existing infrastructure, and very few secondary recovery projects to date, but also great challenges to modeling for maximum sweep efficiencies and recovery factors during miscible CO 2 -EOR projects. In the northern reef trend, a local source for subsequent CO 2 flooding is readily available as a by-product of Antrim Shale production. The annual production of CO 2 separated from Antrim gas is approximately 21 bcf, most of which is currently vented directly into the atmosphere. The close proximity of a source of high-quality CO 2 from several gas-processing plants throughout the northern reef trend, a region with more than 800 Niagaran reef fields, provides an economically viable opportunity to combine CO 2 -flood EOR operations with geological sequestration of CO 2 greenhouse gases. Initial results of a pilot project where CO 2 from the Antrim Shale is being injected into several Niagaran reefs are discussed along with reservoir characterization issues associated with these heterogeneous reservoirs. Similar EOR projects throughout the northern reef trend could provide an economic foundation for CO 2 sequestration programs. This is especially the case if they are designed alongside industrial activities that generate easily captured CO 2 emissions streams, such as other gas-processing plants or future ethanol plants planned for the region.

The pinnacle reefs of the Michigan Basin form small, isolated hydrocarbon reservoirs encased in impermeable evaporites and mudstones, and account for most of Michigan’s hydrocarbon reserves. The temporal relations between the reef sequence and the evaporites are still in dispute, but in the currently favored model, deposition of reefs and evaporites follow each other closely in a cyclic manner but are not synchronous. Pinnacle development occurred in four stages and included periods of subaerial exposure, which enhanced reef porosity and permeability through leaching and dolomitization. Subsequent evaporite precipitation filled much of this porosity; many reefs are completely salt plugged and impermeable. Sea-level history and depositional environments of Salina evaporites are disputed, but a model of shallow-water evaporite deposition in the basin is favored. Regional trends have been recognized across the pinnacle-reef belt, and these predict increased salt plugging of the reefs basinward, increased dolomitization and preserved secondary porosity toward the basin margin, and in the northern trend, zones of production that pass from gas to oil and finally to water toward the basin margin. The producing reefs have porosities that range from 3 to 37 percent (average 6 percent) and average permeabilities of 11 to 12 mD (ranges to more than 1 D).